Common scab is a disease widely distributed in potato-growing areas. Superficial or deep corky lesions on potato tubers characterise the disease. Streptomyces scabies (Lambert and Loria, 1989) is the main causal agent of the disease (Goyer et al., 1996). Methods used to control common scab include chemical treatments of seed potato tubers (Davis et al., 1976), irrigation (Adams and Lapwood, 1978), soil amendments (Weinhold and Brown, 1968), cultivar choice (Bouchek-Mechiche et al., 2000) and rotation strategies (Li et al., 1999).
Biological control of potato scab by nonpathogenic streptomycetes was also reported (Doumbou et al., 2001b; Doumbou et al., 1998; Liu et al., 1995a; Ryan and Kinkel, 1997). For example, introduction into an infested soil of some Streptomyces diastatochromogenes and S. albogriseolus strains isolated from suppressive soils decreased common scab symptoms on potato tubers (Eckwall and Schottel, 1997; Liu et al., 1995a; Lorang et al., 1995). Common scab severity was also decreased by an amendment of an antibiotic biofertilizer produced from swine feces containing S. albidoflavus strain CH-33 (Hayashida et al., 1989). These strains are not known to produce geldanamycin.
Geldanamycin is known to inhibit the growth of several plant pathogenic fungi (Toussaint et al., 1997) and geldanamycin-producing streptomycetes were shown to protect crops against several fungal diseases (Rothrock and Gottlieb, 1984; Valois et al., 1996). This antibiotic is also active against some Gram-positive bacteria (Toussaint et al., 1997) such as S. scabies (Agbessi, 2002) but the efficiency of this bioagent to control common scab has not yet been determined. It cannot be predicted whether a strain known to be antagonistic against a microbiological pathogen will be able to compete in the rhizosphere against this pathogen and against other soil microbiological pathogens. It cannot therefore be predicted whether this strain would be effective in controlling a plant disease caused by this microbiological pathogen.
Certain references have shown that specific chitin waste-based composts suppress common scab (Côté et al., 2001; Vruggink (1970)). It was never demonstrated however whether the little amount of chitosan contained in these wastes participated in controlling the disease. Some success of biocontrol have been reported by the combined use of a biocontrol agent and of chitosan without however demonstrating the role played by chitosan itself (Cuero et al., 1991; Singh et al., 1999). Chitosan is known to be toxic against most microorganisms. There thus remains a need to demonstrate whether chitosan may help control common scab and whether it may be used in combination with microorganisms antagonistic to common scab.
There thus remains a need for new effective biocontrols of common scab.